This invention relates to lead-free, one-coat adhesive compositions capable of bonding rubber to metal without the need of a primer.
As a general rule, most known adhesives shown to be effective as single-coat, i.e., primerless, rubber-to-metal bonding agents are often only useful in bonding specific elastomers to specific substrates and thus are sadly lacking in versatility. This problem of lack of versatility in these adhesives has been partially alleviated by the use of a primer coat applied to the metal substrate and a cover coat which adheres between the elastomer and the primer.
In addition to the problem of versatility most one-coat adhesive systems suffer from one or more other disadvantages including a general inability to afford optimum adhesion, particularly at elevated temperatures, poor storage stability, poor resistance to pre-bake, poor corrosion resistance and poor resistance of the adhesive bond to environmental conditions such as solvents, moisture and the like.
More recently, one-coat compositions have been developed which satisfy these requirements. However, these adhesive compositions require the use of lead compounds to achieve the desired results. For example, U.S. Pat. No. 3,640,941 describes a one-coat rubber-to-metal adhesive containing four essential ingredients: (a) a graft polymer of a polybutadiene and a substituted cyclopentadiene monomer, (b) dibasic lead phosphite, (c) resorcinol and (d) a volatile solvent. In this adhesive system between 25-150 parts by weight of dibasic lead phosphate per 100 parts of polymer is described as necessary to achieve the desired effect.
Another commercially accepted, one-coat adhesive composition is described in U.S. Pat. No. 4,119,587. This patent describes an adhesive composition comprised of the three essential constituents: (a) halogenated polyolefinic, (b) aromatic nitroso compound, and (c) lead salts.
In view of the potential adverse environmental effects posed by the presence of lead, it would be highly desirable if a one-coat rubber-to-metal bonding adhesive composition could be developed which possesses all the aforementioned characteristics without the need of lead compounds.
It is an object of the invention to provide a lead-free, adhesive composition capable of bonding a variety of elastomers to various metal substrates without the need of a primer.
Another object of the invention is to provide a general purpose one-coat adhesive composition exhibiting excellent bond versatility, sweep resistance, storage stability and environmental resistance.
Yet another object of the invention is to provide a one-coat rubber-to-metal bonding adhesive which provides excellent corrosion inhibition.
A further object of the invention is to provide a lead-free adhesive system which affords several handling advantages and enables dry film thickness readings to be taken using equipment that employs beta backscattering techniques.